Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairHard
A technician is performing a fluid flush on a vehicle's brake system. After flushing the system with new fluid, the customer complains of a 'spongy' pedal. The technician re-bleeds the system multiple times, but the spongy feel persists. During the flush, the technician used a pressure bleeder and noticed the fluid level in the master cylinder reservoir dropped critically low at one point. What is the MOST likely cause of the persistent spongy pedal?
- AA faulty residual pressure valve.
- BContaminated brake fluid.
- CA collapsed flexible brake hose.
- DAir introduced into the master cylinder.
Show answer & explanationAnswer & explanation
Correct answer: D. Air introduced into the master cylinder.
If the master cylinder reservoir is allowed to run critically low during a fluid flush, especially with a pressure bleeder, air can be drawn into the master cylinder itself. This air can be very difficult to purge with conventional bleeding, leading to a persistent spongy pedal. The master cylinder might need to be bench bled or a specialized ABS bleed performed if applicable.
Why the other options are wrong
- A. A faulty residual pressure valve would typically cause a hard pedal or no pressure in drum brakes, not a spongy pedal.
- B. Contaminated fluid can cause various issues (e.g., seal degradation, boiling), but not typically a persistent spongy pedal that resists bleeding unless it's boiling at low temps.
- C. A collapsed flexible brake hose would cause a drag or pull to one side, not a general spongy pedal throughout the system.
Master Cylinder Air Intrusion
Air entering the master cylinder's internal circuits, usually due to the reservoir running dry, making subsequent bleeding difficult and causing a spongy pedal.
- Occurs if reservoir runs dry during bleeding/flush.
- Causes persistent spongy pedal.
- Often requires bench bleeding or specialized procedure to remove.
Memory trick: Low fluid, air flew, spongy pedal, what to do?